1. Field of the Invention
The present invention relates generally to a video detector, and more particularly, to a video detector employing a PLL (Phase Locked Loop) system contained in an apparatus comprising a function of receiving a television signal, such as a television (referred to as TV hereinafter) receiver and a home video tape recorder (referred to as VTR hereinafter).
2. Description of the Prior Art
Conventionally, in an apparatus comprising a function of receiving a TV signal, such as a TV receiver and a VTR, a PLL system has been employed as a video detector circuit. More specifically, if such a video detector circuit employing the PLL system is used, a buzz sound caused by inclusion of a video signal in a sound signal in a transmission system of the TV signal can be reduced as well as differential gain (DG) and differential phase (DP) can be improved, so that a stable color image can be reproduced. Such a video detector circuit employing the PLL system is disclosed in, for example, Japanese Patent Laying-Open Gazette Nos. 210771/1983, 24784/1985 and 198882/1986.
FIG. 1 is a block diagram showing an example of a conventional TV signal receiving circuit using such a PLL video detector circuit.
Description is now made on structure of the circuit shown in FIG. 1. In FIG. 1, a TV signal received by a TV receiving antenna 1 and a TV tuner 2 is applied to a first surface wave filter 3 which is a bandpass filter enabling the passage of a signal component of 45.75 MHz and a second surface wave filter 4 which is a bandpass filter enabling the passage of a signal component of 41.25 MHz. A video intermediate frequency signal of 45.75 MHz extracted by the first surface wave filter 3 is applied to a first intermediate frequency amplifier circuit 5 while a sound intermediate frequency signal of 41.25 MHz extracted by the second surface wave filter 4 is applied to a second intermediate frequency amplifier circuit 6.
An output of the first intermediate frequency amplifier circuit 5 is applied to respective one inputs of a synchronous detector circuit 7 and a phase detector circuit 8. On the other hand, an oscillation output of a voltage controlled oscillator (referred to as VCO hereinafter) 9 which oscillates at the video intermediate frequency of 45.75 MHz is applied to the other input of the phase detector circuit 8. The phase detector circuit 8 outputs a signal corresponding to the phase difference between both the input signals to apply the same to a low-pass filter 10. The low-pass filter 10 smoothes an output of the phase detector circuit 8 to apply the same to the VCO 9, thereby to control the phase of the oscillation output of the VCO 9. The phase detector circuit 8, the VCO 9 and the low-pass filter 10 constitute a PLL portion 11. In addition, the oscillation output of 45.75 MHz of the VCO 9 is applied to the other input of the synchronous detector circuit 7 after shifting the phase thereof by 90.degree. through a 90.degree. phase sifter 12. A video signal synchronously detected by the synchronous detector circuit 7 is applied to a video signal processor circuit 14 after removal of the sound signal component thereof through a sound trap circuit 13. A video output from the video signal processor circuit 14 is outputted to the exterior through a video output terminal 15, to be applied to a monitor receiver or the like.
On the other hand, the sound intermediate frequency signal of 41.25 MHz outputted from the second intermediate frequency amplifier circuit 6 is applied to one input of a multiplier 16. In addition, the output of 45.75 MHz of the 90.degree. phase shifter 12 is applied to the other input of the multiplier 16. An output of the multiplier 16 is applied to a bandpass filter 17 enabling the passage of a signal component of 4.5 MHz. An FM sound signal having a frequency of the difference between the video intermediate frequency of 45.75 MHz and the sound intermediate frequency of 41.25 MHz, i.e., a frequency of 4.5 MHz is extracted from the bandpass filter 17, to be applied to an FM detector circuit 18. A sound output from the FM detector circuit 18 is outputted to the exterior through a sound output terminal 19, to be applied to the monitor receiver or the like.
Description is now made on an operation of the conventional circuit shown in FIG. 1. In the circuit shown in FIG. 1, the video intermediate frequency signal of 45.75 MHz amplified by the first intermediate frequency amplifier circuit 5 and the oscillation output of 45.75 MHz of the VCO 9 are phase-detected by the phase detector circuit 8. The output of the phase detector circuit 8 corresponding to the phase difference therebetween is smoothed by the low-pass filter 10. The output of the low-pass filter 10 controls the phase of the oscillation output of the VCO 9 such that the phase difference between the video intermediate frequency signal and the oscillation output of the VCO 9 becomes 90.degree.. The oscillation output of the VCO 9 is made in phase with the video intermediate frequency signal by shifting the phase thereof by 90.degree. through the 90.degree. phase shifter 12 and then, applied to the synchronous detector circuit 7 and the multiplier 16. The synchronous detector circuit 7 responsively detects synchronously the video intermediate frequency signal, to apply the detected video signal to the video signal processor circuit 14 through the sound trap circuit 13.
Assuming that the video signal is represented by; EQU V.sub.V =A.sub.O kcos (pt+.theta.)
the carrier thereof is represented by; EQU V.sub.C =A.sub.O cos (Wct+.phi..sub.O)
a video intermediate frequency signal e.sub.IF obtained through amplitude modulation is as follows; ##EQU1## where k is the modulation degree.
The phase detector circuit 8 is responsive to the video intermediate frequency signal e.sub.IF and a signal V=cos (Wct+.PSI.) having the video intermediate frequency outputted from the VCO 9 for outputting a signal e.sub.PD which is the product of both the signals; ##EQU2##
The output signal e.sub.PD is passed through the low-pass filter 10, whereby a frequency component of approximately 2 Wc is removed. More specifically, an output signal e.sub.LPF of the low-pass filter 10 is as follows: ##EQU3##
Considering the ideal state in which capture of the PLL is achieved, the phase difference (.phi..sub.O -.PSI.) between the output of the VCO 9 and the video intermediate frequency signal is 90.degree.. Thus, the output signal e.sub.LPF of the low-pass filter 10 becomes 0 V. As a result, the VCO 9 stably oscillates.
However, the video intermediate frequency signal is actually subjected to amplitude distortion and phase distortion because it passes through the first surface wave filter 3 provided on the input side of the first intermediate frequency amplifier circuit 5.
More specifically, the video intermediate frequency signal e.sub.IF inputted to the phase detector circuit 8 is subjected to the amplitude distortion and the phase distortion caused by the filter 3, actually to be a signal e'.sub.IF as shown below: ##EQU4## where Y.sub.0 is the attenuation constant, and .DELTA..phi..sub.+p, .DELTA..phi..sub.-p is phase distortion. Thus, the signal e.sub.LPF responsively outputted from the low-pass filter 10 is affected by such amplitude distortion and phase distortion, actually to be a signal e'.sub.LPF as shown below: ##EQU5##
Thus, even if the phase difference (.phi..sub.0 -.PSI.) between the output of the VCO 9 and the video intermediate frequency signal is 90.degree. as described above, the second term and the third term in the signal e'.sub.LPF remain. Consequently, in order to achieve e'.sub.LPF =0, it is necessary to satisfy the following condition: EQU Y.sub.+p =Y.sub.-p and .DELTA..phi..sub.+p =.DELTA..phi..sub.-p
FIG. 2 is a diagram showing bandwidth characteristics of the surface wave filter 3 shown in FIG. 1, where the axis of abscissa represents the angular velocity of the input signal and the axis of ordinate represents the amount of attenuation of the output signal. In the actual video detector circuit, a signal transmitted employing a vestigial sideband system is processed, so that amplitude characteristics and group delay characteristics of the above described surface wave filter can not be made flat in the vicinity of Wc, as shown in FIG. 2. Thus, since the above described condition Y.sub.+p =y.sub.-p and .DELTA..phi..sub.+p =.DELTA..phi..sub.-p can not be achieved, the video intermediate frequency signal e'.sub.IF receives the distortion as shown in the positions of the angular velocities Wc, Wc+P and Wc-P. Thus, e'.sub.LPF =0 can not be achieved, so that stable oscillation of the VCO 9 is prevented.
The signal e'.sub.LPF having an angular velocity of pt outputted from the low-pass filter 10 particularly includes many signal components which are horizontal frequency components (f.sub.H, 2f.sub.H, . . . ) in the video signal as unnecessary components. Such unnecessary video signal components are the cause of a grating buzz sound in the sound output as described below.
It is assumed that sound multiplexing broadcasting in Japan is received. In this case, a subchannel signal is demodulated by amplifying a composite sound signal detected by an SIF circuit to limit the amplitude thereof and then, extracting a 2f.sub.H component, i.e., a subcarrier of the subchannel signal by the bandpass filter to frequency-modulate the same. However, a f.sub.H component among the above described unnecessary components included in the output of the low-pass filter generates a secondary higher harmonic of 2f.sub.H at the time of amplification and limitation of the amplitude. The secondary higher harmonic of 2f.sub.H is superposed on the subcarrier of 2f.sub.H of the composite sound signal. As a result, a so-called stereo buzz is produced.
In addition, it is assumed that sound multiplexing broadcasting in the United States is received. In this case, the 2f.sub.H component among the above described unnecessary components included in the output of the low-pass filter is included in the subchannel signal in which a carrier of 2f.sub.H is suppressed to be transmitted employing a double sideband transmission system. As a result, at the time of demodulation, beat is produced between a carrier component of 2f.sub.H created for restoring the above described suppressed carrier and the above described unnecessary component of 2f.sub.H. This beat degrades the distortion factor of stereo demodulation, which causes the buzz sound.
As described in the foregoing, since the unnecessary components in the output of the low-pass filter 10 shown in FIG. 1 prevent stable oscillation of the VCO 9, the unnecessary components significantly adversely affect the sound output as described above, while adversely affecting the differential gain and the differential phase, resulting in degradation of the picture quality such as the change of the color phase.
Additionally, in order to prevent the above described adverse effect, a method for increasing the time constant of the low-pass filter 10 to reduce the unnecessary components is considered. However, the capture range of the PLL becomes narrow, so that this method can not be employed.